1. Field of the Invention
The present invention relates to a process for aseptically packing water-containing products, which are subjected to a heat treatment in the package.
In the aseptic packaging of foodstuffs such as e.g. bread, and other water-containing products which are heat-treated in the package to effect decontamination, the principal and most obvious problem is to avoid in film packages recontamination when the heated package is cooled down. Provision must be made to release gas generated during the heating step to avoid the package bursting or expanding unduly, and packaging material may be chosen for this purpose which is pervious to gas and water vapour during the heating operation. Alternatively, valve means may be fitted to otherwise impervious packages which opens during the heating step. Whatever means are adopted, condensation and/or contraction of vapour remaining in the package may lead to the package collapsing on its contents and will in any case tend, as soon as a leak develops, to contaminate the contents of the package by sucking in airborne infective micro-organisms. If, on the other hand, air is deliberately admitted during the cooling step to equalise the pressure inside and outside the package, then the danger of infection again occurs. Hitherto it has appeared that simple discrete vents remaining open during both the heating and cooling steps could not be adopted for these reasons.
The present invention is based upon the discovery that the security of the sealed package is ensured if the vents are installed as interruptions in a seam first made in the package to enclose the goods therein.
2. Prior Art
In British Pat. No. 852,946 a bag is described made from plastic material impervious to water and oxygen which is provided with overlapping lips to close the bag hermetically under normal storage but to open under internal pressure generated by the release of carbon dioxide, preventing the bag from bursting when yeast is stored in it.
British Pat. No. 887,215 describes a bag furnished with a thread extending through two heat-sealed positions to provide vents which will release pressure generated when the bag is heated. A container lid for a cup or carton is described in British Pat. No. 917,998, having vent opening means through the lid covered with vapour-pervious sheet material to allow steam to escape from the container while retaining potable liquids held in the container. A different arrangement for similar containers is described in British Pat. No. 934,067, a sanitary seal being provided between the lid and the container, the parts of which open under excess pressure generated within the container to release gas therefrom. In British Pat. No. 951,228 an interrupted seal is formed across the opening of a plastics bag which is small enough to retain granular solids and similar small articles, while permitting air to be expelled from the bag to improve the pallet stability of the filled bag or sack.
In British Pat. No. 961,821 a packaging film is described which is embossed to provide raised portions. These are pierced to permit the passage of gas.
The ingress of bacteria into packages is prevented according to British Pat. No. 1,006,678 by means of a backing layer made for example of paper on a synthetic resin such as polyethylene, the packing omitting the permeation of oxygen. In British Pat. No. 1,021,596 a container is fitted with a snap-on closure forming an annular seal which is temporarily broken when pressure is generated in the container so that gases may escape via flutes in the upper side wall of the container. In British Pat. No. 1,106,747 a film of shrink-type material covering paper or plastics receptacles is punctured by needles to permit escape of gases. According to British Pat. No. 1,192,751, small perforations are provided in a metal foil package permitting the escape of gases from inside the package but being too small to permit atmospheric oxygen to enter.
In none of the arrangements described in the prior art is reliance placed upon seam vents remaining open during the cooling period after heating containers to sterilise their contents.
The packaging of bread is described in British Pat. No. 1,008,679, according to which it is wrapped while still hot from the oven in a material which becomes impervious to water below 25.degree. C. According to British Pat. No. 1,197,130, a channel is left between adjacent packs used to package bread which permits the escape of gases during sterilisation by heat treatment, the channel being sealed when the packaging operation is completed.
In most of the prior art disclosure is confined either to temporary passageways which are sealed in the final packaging operation, or which use comparatively porous material. Packages intended to prevent the ingress of bacteria are confined, as in British Pat. No. 1,006,678, to the use of porous film rather than vent orifices. Those disclosures of small vents are confined to holes through the thickness of film material.